1. Field of the Invention
This invention relates to a process and apparatus for manufacturing polycrystalline silicon and a process for manufacturing a silicon wafer for a solar cell. In particular, this invention pertains to a technique which employs metallic silicon or silicon oxide as a starting material and permits the continuous flow production from polycrystalline silicon to an end product, that is, a polycrystalline silicon wafer for a solar cell.
2. Description of the Related Art
Studies on solar cells have been made for many years. Recently, those having a photoelectric transfer efficiency of even about 13 to 15% under sun light on the ground have appeared and they are now being industrialized for various applications. In our country, however, solar cells are not so popular as an energy source for domestic electric power, automobiles, ships or machine tools, because a technique to mass-produce a silicon wafer at a low cost, which is necessary for the manufacture of solar cells, has not yet been established.
At present, for the manufacture of a silicon wafer for a solar cell, a high-purity silicon, which is in the mass form and conforms to the specification of a semiconductor, is once manufactured through a chemical process by using as a starting material a low purity metallic silicon (99.5 wt. % Si). Then, high-purity silicon in the mass form is re-melted and is adjusted to have a chemical composition suited to a solar cell by a metallurgical process. The resulting molten silicon is formed into an ingot by the pulling method or directional solidification method, followed by slicing into thin plates. Described specifically, as shown in FIG. 5, metallic silicon is first reacted with hydrochloric acid and formed into a trichlorosilane gas. After the gas so obtained is fractionated to remove the impurity elements, the residue is reacted with a hydrogen gas, whereby high-purity silicon is precipitated from the gas by the so-called CVD (Chemical Vapor Deposition) method. The high-purity silicon therefore becomes only an aggregate of silicon grains owing to the weak bonding power between crystal grains. The boron contained in the high-purity silicon forming the aggregate is reduced even in the order of 0.001 ppm and does not reach the concentration necessary for satisfying the specific resistivity of 0.5 to 1.5 ohm.cm which is the specification for P-type semiconductor wafer. In order to use the above high-purity silicon for a solar cell, it is indispensable to adjust the specific resistivity and to control the crystallinity of single crystals or crystal grains so as to have a particle size not smaller than several mm and have a grain boundary so as not to exert adverse effects on the photoelectric transfer efficiency. The above silicon cannot be formed into a wafer without further treatment. As shown in the right hand of FIG. 5, it becomes necessary to form a wafer after re-melting the high-purity silicon mass, adjusting the components of the melt (by the addition of boron) and forming into an ingot (pulling method for single crystals, while directional solidification for polycrystals).
The above-described manufacturing method is however accompanied with the drawbacks that it requires much labor to re-adjust (mainly, by the addition of boron) the components of a silicon ingot, which has a purity intentionally heightened to be suitable for semiconductor, to be suitable for solar cells or to purify the ingot; its yield is inferior; it additionally requires equipment and energy for re-melting; and therefore, it is expensive. As described above, the solar cells available now are therefore expensive, which prevents them from being popularly used. The purity heightening of metallic silicon by a chemical process is also accompanied by the generation of a large amount of pollutants such as silane and chloride, which prevents mass-production. According to the above described technique, the manufacturing process tends to be studied, divided into steps such as increasing the purity of metallic silicon, or using the solidification technique, which is presumed to be influenced by the above-described manufacturing method.
For example, Japanese Published Unexamined Patent Application No. HEI 5-139713 discloses a process in which silicon having a low boron content is obtained by maintaining molten silicon in a container composed of silica or composed mainly of silica, and injecting a plasma gas jet flow of an inert gas to the surface of the molten silicon, while blowing an inert gas upwardly from the bottom of the container. Japanese Published Unexamined Patent Application No. HEI 7-17704 discloses a process permitting the efficient removal of boron by forming 1.5 to 15 kg of SiO.sub.2 per kg silicon in advance on the surface of metallic silicon powders upon melting metallic silicon through an electron beam. Concerning solidification technique, Japanese Published Unexamined Patent Application No. SHO 61-141612 proposes a technique to prevent, upon casting molten silicon into a mold, precipitation of inclusion in a silicon ingot by turning the mold. In addition, the present applicants themselves are now proposing a method for purifying molten metallic silicon by directional solidification in Japanese Patent Application HEI 7-29500 (filed on Feb. 17, 1995).
It is impossible to say that there does not exist a technique to manufacture solar cell silicon directly from metallic silicon. For example, Japanese Published Unexamined Patent Application No. SHO 62-252393 discloses a process in which a starting material silicon, which is once used as a semiconductor but disposed as an electron industry waste, is subjected to zone melting by plasma jet generated by a mixed gas of argon, hydrogen and oxygen. This process aims principally at the use of an industrial waste so that it does not become a mainly-employed technique suited for mass production of a silicon wafer. In addition, although silicon is used as a raw material, its purity has been once increased so that the process is only a variation of the above-described cumbersome manufacturing process. Japanese Published Unexamined Patent Application No. SHO 63-218506 discloses a process for manufacturing, by plasma melting, silicon in the mass form for solar cells or electronics from metallic silicon in the form of powders, granules or polished dusts. This method is based on the principle of the zone melting method using the same plasma as that disclosed in the above Japanese Published Unexamined Patent Application No. SHO 62-252393 and is accompanied with the drawback that mass production cannot be carried out in spite of large electricity consumption. According to Examples of the above official gazette, only a silicon rod of 50 g or so is obtained on a laboratory scale and it does not include a description of increasing the size of the silicon wafer for a solar cell to a practical size.